Emit C++ string literals for HILTI string literals.

When emitting literals for HILTI strings (string ctors) we would
previously explicitly force creation of `std::string`. This was almost
always an unnecessary pessimisation over emitting string literals since
even if their C++ uses expected `std::string` string literals can
convert to this type implicitly; at the same time it made it impossible
to make effective use of APIs accepting `std::string_view`.

With this patch we now emit C++ string literals for HILTI string
literals.

hilti/toolchain/src/compiler/codegen/ctors.cc

@@ -181,7 +181,21 @@ struct Visitor : hilti::visitor::PreOrder<cxx::Expression, Visitor> {
         return fmt("::hilti::rt::Stream(\"%s\"_b)", util::escapeBytesForCxx(n.value()));
     }
 
-    result_t operator()(const ctor::String& n) { return fmt("std::string(\"%s\")", util::escapeUTF8(n.value(), true)); }
+    result_t operator()(const ctor::String& n) {
+        // We emit string constructors for both user-provided literals and
+        // generated strings, e.g., static error messages or identifiers. Only
+        // user-provided literals need to behave like actual our runtime string
+        // type `std::string` since they e.g., need to be usable with functions
+        // expecting that type or might need to be mutable; generated string
+        // OTOH are static and only used with our internal APIs which we can
+        // make usable with actual literals, and we can avoid allocating owning
+        // mutable strings for them.
+        bool is_spelled = n.meta().location().file().empty();
+        if ( is_spelled )
+            return fmt("std::string_view(\"%s\")", util::escapeUTF8(n.value(), true));
+        else
+            return fmt("std::string(\"%s\")", util::escapeUTF8(n.value(), true));
+    }
 
     result_t operator()(const ctor::Tuple& n) {
         return fmt("std::make_tuple(%s)",